According to a study (Deyo et al., 2006), about one fourth of U.S. adults report low back pain in the past 3 months, emphasizing the prevalence and importance of this condition. It is generally accepted that prolonged, static sitting postures are likely to aggravate a preexisting low back condition or instigate the development of a new condition (Kelsey & White, 1980; Lis et al., 2007; Battié´ et al., 2002). Also, a recent occupational study finds that sitting hours positively correlate with lower back pain (Mendelek et al., 2011). In sitting, the pelvis rotates posteriorly and the lumbar lordosis (the natural curve of the lower back) flattens (De Carvalho et al., 2010). This posture increases the strain on the posterior passive elements of the spine (De Carvalho et al., 2010; O'Sullivan et al., 2011; Lengsfeld et al., 2000). The problem stems from the very fact that chair sitting requires raising both upper legs. This reduces the angle between the thigh and the torso from 180 degrees to near 90 degrees therefore pushing the posterior passive elements of the spine (De Carvalho et al., 2010).
There are some claver chair products increasing the thigh-torso angle by providing a knee support and a forward-tilted seat (U.S. Pat. Nos. 4,765,684; 5,782,534), thereby reducing back pain. Another category of products is standing chair (U.S. Pat. Nos. 3,704,847; 4,738,487), by which the user maintains a half-standing posture; sitting on a raised, tilted-forward seat. There are other inventions which let the user change the thigh-torso angle by having two split half seats individually tilt forward (U.S. Pat. Nos. 4,451,086; 7,387,339 B2) or mobilizing the pelvis (U.S. Pat. Nos. 6,139,095; 6,357,827 B1) while the user is in a sitting position. While these products increase the spine-thigh angle, they do not reach the ideal 180 degrees (standing posture); and the postures that the user needs to maintain are rather unnatural.